The invention relates to a fuel cell stack comprising: a plurality of fuel cells electrically connected in series and having equivalent active section areas and circumferences, each fuel cell comprising a laminate of layers, these comprising an electrolyte membrane and catalyst, electrode and gas diffusion layers which functions may be combined in any combination in multifunction layers; end and separator plates delimiting each cell; and cooling layers the cooling function of which also may be combined with other l in respective multifunction layers, the cooling layers each projecting beyond the circumferential outer periphery of the laminate of the other layers thereby an inner active area and a peripheral cooling fin area and the ratio of the of circumference and active area of the fuel cells, defined by the geometrical shape of the active area, being greater than the corresponding ratio of a square active area. Such a sort of stack can be derived from U.S. Pat. No. 3,589,942 A.
The need for a cooling system in a hydrogen-air proton exchange membrane (PEM) fuel cell has been a long-standing problem. If the heat rises to a very high temperature level, there exists the danger of drying out the membrane. That results in a loss of ionic conductivity and performance, because of the membrane""s need of a high humidity level.
Therefore, fuel cells which have a considerable heat dissipation need a cooling system. For certain applications it is helpful that the cooling system is lightweight and compact, e g. for a mobile system. In all fuel cell applications the cost factor of an additional peripheral system for the fuel cell system is to be considered.
In U.S. Pat. No. 5,595,834 A, a fuel cell stack is described that has a circular cylindrical shape. For cooling, it uses separator plates that extend radially outward from the periphery of the stack to serve the additional function of cooling fins. However, the heat transfer to the cooling fins and further to an air stream is restricted which is a considerable limitation particularly for larger cell areas and high temperature gradients and weights. That is because the cooling area of the ring-shaped cooling fins is not sufficient unless using unduly thick layers and broad rings with the consequence of a large overall weight and volume. Rather, the design of the prior art provides relatively large active section areas where waste heat is produced and only relatively small cross sectional areas to conduct the heat outside, and relatively restricted cooling fin areas. An additional cooling system will be necessary that increases the peripheral aggregates of the total system.
U.S. Pat. No. 5,776,625 A and WO 98/11616 describe similar heat removal schemes. The bipolar plates extend in length or width over the active area of each single cell of the fuel cell stack, forming fins at opposing stack edges or at one stack side, respectively. The concomitant increase in stack surface area allows the stack to be cooled e. g. via air that is blown across its surface by a fan.
U.S. Pat. No. 3,589,942 A mentioned above, in its drawings, shows a fuel cell stack wherein, seen from the perspective of the prism shaped stack, the ratio of the circumference to the active section area of the stack, defined by the geometrical shape of the active section area, exceeds the corresponding ratio of a square base prism, i.e. of the transversal section of a straight parallelepiped with a square base, with the same section area. This provides for a relatively longer circumferential line around each cell and further for a larger fin area in relation to the fin width, with the consequence of a lower heat conduction resistance and a higher heat convection.
However still in the prior art, the ratio of the heat dissipation ability to the heat generation in the active area is not optimal, particularly if using thin and lightweight heat conducting layers. A need exists for a cooling system for fuel cell stacks that is inexpensive, lightweight, compact, and conducts all the waste heat of the reaction process out of the fuel cell stack.
This invention relates to fuel cell stacks as defined above, and preferably to those composed of air breathing proton exchange membrane fuel cells, more particularly polymer electrolyte membrane fuel cells, that operate with air as a reactant and cooling gas and have cooling layers comprising the external cooling fins. The cooling layers may be part of the single fuel cells and extend parallelly to the flat fuel cell extension. The purpose is to bring out the total heat of the reaction process, at first from the inside of the stack to the cooling fins and further from the cooling fins to ambient air or another cooling fluid.
According to the invention, the material for use in the cooling layer is a foil made of expanded graphite. This material if also used as a gas diffusion material may be mixed with soot. Expanded graphite is known to be a useful material for the electrodes of the fuel cell, for the gas diffusion layer or flow field and for the separator plates (see e.g. EP 0 784 352 A). If used for the cooling layers, the feature becomes important that such foil has a larger specific heat conductivity parallel to the plane of each fuel cell, relative to the density, than the metals, since it is a rather light material. The rather poor heat conductivity orthogonal to that plane is of almost no relevance.
The cooling layer may be an extension of the anode or cathode flow field or of the bipolar plate or may even be an extension of one electrode. Preferably it covers the whole active area.
The geometrical form of the fuel cells, being members of the fuel cell stack or another configuration of one or more cells itself is preferably rectangular, and preferably the length is about 1.5 times the width. More preferaably, the length is 2.5 to 3.5 times the width. It at least has a geometric shape that has a relatively high circumference compared to the area. With this configuration, the distance that the total waste heat transport has to cover is short, and the cross-section depending on the thickness and the extension of the expanded graphite cooling layer, to conduct the heat outside is high enough even if rather thin cooling layers are used. The smaller distance of the rectangular fuel cell is used as a pathway for the heat. The result of this material and shape is that the cell, especially the cooling layer, has less weight than if conventional shapes, like a circle or a square, and/or metal cooling fins are used. And as mentioned, for an equal extending width, the circumferential cooling fin has a larger area for dissipating the heat.
Although not necessary, it will be assumed favourable to use the separator plates, i.e. the bipolar plates separating the single cells in the stack, for the cooling plates. However, there is some difficulty in the structure of the bipolar plate. The bipolar plate must have enough electric conductivity orthogonal to the plane of the fuel cell That is one reason why most fuel cells use a graphite or metal plate as a bipolar plate. But in order to use the bipolar plate as a cooling fin, the heat (or nearby equivalently the electrical) conductivity parallel to the plane of the fuel cell must be enlarged to bring out all the waste heat of the reaction processes. Therefore, the present invention uses the foil containing expanded graphite that has extremely anisotropic features concerning heat and electrical conductivity. The heat conductivity may be fifty (50) times lager parallel to the sheet plane than orthogonal to that plane.